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Solar power. Everyone would love to have it, but it's too expensive, intermittent, or otherwise difficult for many applications. What are some things that can be done with solar? What are the limits?
40 responses total.
One of the most practical solar devices is the solar water heater. They're relatively inexpensive and quite effective. The "water pillow" style of heater is available at camping stores for a few dollars.
Solar power is the power source of choice for remote radio, telemetry, data logging, even telephone, devices.
True. Any device a long way from power lines, and requiring less than a few hundred watts, is an excellent candidate for photovoltaic (PV) power. The PV panels are expensive, but cheaper than running wires long distances.
My main calculator for the past decade or so is powered by "solar" energy (is it still solar power you use electric lights to power it?) I do sometimes have to turn on a light to use it, but overall I prefer it to replacing/recharging batteries.
(I've read that if you calculate the cost of the power from the cells in a solar-powered calculator, you get a figure in the neighborhood of $50/KWH (yes, fifty dollars per KWH). Given the negligible power consumption of CMOS circuitry, and the freedom from batteries, it's still a pretty good deal.)
With only an initial cost, shouldn't the cost per kwh go down the more you use it? Or did that assume some finite usage period? For regular batteries, I could see coming up with a single figure like that...I bet that's high compared to Detroit Edison, too.
The house I'm designing will use solar energy to help heat it in the winter. I'm also considering a solar powered hot water preheater. Converting solar enery to electricity is only about 10% efficent at the time and costs are around $4 / watt. I considered a deep well pump that only required a 300 watt panel to operate. The pump was $1500 and the panel to power it would be $1200. Not too good a choice when you have power available on your site, but for situations where the utility wants several grand to bring in power...
Re #6: I don't recall. It was an interesting number. Coin cells are probably in that range too.
I have been working some numbers relating to efficiency of solar-thermal (steam) engines. What I've been getting has surprised me. Unless I am making very unrealistic assumptions, it should be feasible to build a solar-steam engine with almost 20% efficiency. It should also be feasible to accept somewhat lower efficiency and run some other device off the discharge steam. If anyone else speaks thermodynamics, I'll post my numbers here.
(I should note here that the roof of a 1000 ft^2 house in Arizona receives almost 100 kilowatts of solar heat on a sunny summer day. Capturing half of that and converting it to electricity at 20% efficiency is 10 KW electrical, or about a buck an hour at retail rates. Such a rig could pay for itself at a rate of about $3000/year over and above providing hot water for almost continuous showers.)
That's a pretty good return on investment. You make and interesting point Russ. I don't think I've ever seen solar cells backed with water laden cooling coils. To me it seems to be a natural to get hot water and electricty off one panel.
You can't do that with silicon solar cells, because the voltage drops quite rapidly with increasing temperature. And PV scales *very* poorly because every square inch of cell costs about the same. Mirrors, pistons and pumps cost not very much more for larger ones as smaller ones; cost depends largely on parts count. This is why I think solar-steam is the future in suny areas.
I speak thermodynamics, so post away.
This is an outgrowth of a discussion I was having elsewhere. To wit,
could a solar-steam engine possibly break 15% thermal efficiency,
given some not-unreasonable assumptions? (The caveat I entered was
that the output steam had to be at 450 F or greater temperature.
The other restriction is that the steam must not be allowed to have
excessive proportions of liquid water.)
Terms for the glossary:
Internal energy: The energy of the molecular motion of the working fluid.
Enthalpy: The internal energy of the working fluid, plus the
product of its pressure and volume (which has the units of energy).
This is used when the fluid crosses boundaries.
Entropy: A measure of the disorder of the working fluid. The units
of entropy are always (energy)/(absolute temperature)
All the above quantities are measured relative to some reference state,
which is arbitrarily assigned the value of zero. In the case of the
steam tables, that is usually the triple point of water. The quantities
are abbreviated: h = enthalpy, s = entropy, u = internal energy (not
used below). When mixtures of liquid and vapor are being handled, the
"quality" (mass-fraction which is vapor) is abbreviated x.
Here are 3 scenarios. In each one, I assume that the expander
(piston or turbine) yields 60% of the available work as output,
the remaining 40% being lost to friction or heat transfer. I
neglected heat loss (insulation is cheap and losses are too
dependent on physical dimensions). I am assuming two expansions
with the steam being re-heated between expansions.
Units: pressure in pounds per square inch absolute.
Enthalpy in BTU per pound-mass.
Entropy in BTU per pound-mass per degree Rankine
(degrees Fahrenheit above absolute zero).
Unit conversions: A BTU (British thermal unit) is the energy
required to heat 1 lbm of water by one degree F. It is
equal to 1054.4 Joules (1 Joule = 1 Watt-second) or
approximately 778 foot-pounds.
Scenario 1: 450 F boiler @ 250 psia, 3 psia condenser (141 F
saturation temperature).
State 0: boiler feedwater @141 F, h = 109.39 BTU/lbm
(This is the water pumped into the boiler.)
State 1: boiler output @450 F, 250 psia. h = 1233.7 s = 1.5632
(Between the feedwater pump and the boiler, 1124.3 BTU/lbm
were added to the steam. About 99.9% of this is heat in
the boiler and 0.1% is work done by the feedwater pump.)
State 2: first expander output @ 60 psia, saturated. 60% of the
available energy is recovered, so h = 1163.7, x = 0.984
(about 1.6% liquid in the mixture), s = 1.6252. The work
done is 70.0 BTU/lbm.
State 3: Re-heater output @ 60 psia, 450 F. h = 1258.3, s = 1.7413.
94.6 BTU/lbm are added in the re-heater.
State 4: Output of the second expander @ 3 psia. h = 1124.6
(slightly superheated). I did not calculate the entropy.
The work done in the second expansion is 133.7 BTU/lbm.
The working fluid is returned to state 0 in the condenser, giving
up 1015.2 BTU/lbm. This closes the cycle. Entropy is greater at
each stage than the last, so the second law (entropy cannot
decrease without rejecting heat) is satisfied.
Total heat input: 1124.3 + 94.6 = 1218.9 BTU/lbm.
Total work output: 70.0 + 133.7 = 203.7 BTU/lbm = 16.7% of total
Total heat rejected = 1124.6 - 109.4 = 1015.2 = 83.3% of total
So it appears that 15% can be beaten.
But that's not the best that can be done. A goodly part of the
heat added in the boiler is spent to bring the feedwater from
141 F up to the boiling point. Adding heat at low temperatures
adds more entropy than adding it at high temperatures. If some
of this heat could be taken from a source that is *already* at a
lower temperature, then this entropy isn't created, just moved.
This decreases the amount of entropy (and thus heat) that has to
be rejected at the condenser, increasing the amount that can be
output as work and thus the efficiency.
One of the tricks in the steam trade is the "feedwater heater".
This taps off low-pressure steam and uses it to heat the water
going into the boiler. The transfer of heat from steam to
water increases entropy, but not as much as adding the same
heat in the boiler because the temperature difference is smaller.
Re-running the previous analysis with a feedwater heater tapping
steam after the first expansion:
State 0: Feedwater to the feedwater heater. 141 F, h = 109.39.
State 1: Output of the feedwater heater. The saturation
temperature of steam at 60 psia is 293 F. Assuming
that the feedwater actually achieves this temperature
the output water will have h = 262.25. It picks up
153.86 BTU/lbm.
State 2: Output of the boiler, 250 psia, 450 F. h = 1233.7, s = 1.6252
State 3: first expander output @ 60 psia, saturated. h = 1163.7,
x = 0.984, s = 1.6252. The work done is 70.0 BTU/lbm.
At this point, enough steam is tapped off to heat up the incoming
water to 293 F. This requires 153.86 / (1163.7 - 109.39) = 0.146
of the total flow, and is condensed in the process; it goes directly
to the feedwater pump. The remainder (85.4%) goes to the reheater
and the second expander.
State 4: output of the reheater, 60 psia, 450 F, h = 1258.3, s = 1.7413.
The heat added is 94.6 BTU/lbm of steam, or (94.6*0.854)=80.8 BTU
of feedwater.
State 5: Output of the second expander, 3 psia, h = 1124.6. The work
output is 133.7 BTU/lbm of steam or (0.854*133.7) = 114.2 BTU/lbm
of feedwater.
The second expander outputs to the condenser, where the remaining steam
is returned to the 141 F liquid state. The heat rejected is 1015.2
BTU/lbm of steam, or 867.0 BTU/lbm of feedwater.
Total heat input: (1233.7 - 262.25 + 80.8) = 1052.2 BTU/lbm
Total work output: 70 + 114.2 = 184.2 BTU/lbm = 17.5%
Total heat rejected: 867.0 BTU/lbm = 82.5% (more or less).
This picks up another 0.8 percent... not really impressive. Something
else has to be done to get big improvements. If the cost of the mirror
and boiler is the major expense of the system, it may pay to get fancier.
The saturation pressure of water at 450 F is 422 psia. It's clear that
the boiler pressure can be increased quite a bit, which also increases
the temperature at which the water boils and decreases the entropy input
(delta_s = delta_h / T). Increasing the boiler pressure looks like it
could improve the efficiency. So could dropping the condenser temperature
and pressure. So I checked the effects of boosting the boiler pressure to
300 psia and dropping the condenser temperature to 120 F (saturation
pressure 1.69 psia). I kept the feedwater heater.
State 0: Condenser output. 120 F, 1.69 psia, h = 88.0 BTU/lbm.
State 1: Output of the feedwater heater. h = 262.25.
State 2: Boiler output. 450 F, 300 psia, h = 1226.2, s = 1.5365
State 3: First expander output. 293 F, 60 psia, h = 1148.6,
s = 1.6052. The work output is 77.6 BTU/lbm.
At this point, steam is tapped for the feedwater heater. The fraction
removed is 174.25/(1148.6-88.0) = 16.4%. 83.6% goes to the reheater.
State 4: Output of the reheater. p = 60 psia, T = 450 F, h = 1258.3, etc.
109.7 BTU/lbm steam is added, or 91.7 BTU/lbm feedwater.
State 5: Output of the second expander. P = 1.69 psia, T = 120 F,
h = 1104.5, x = 0.991. The work output is 153.8 BTU/lbm steam
or 128.6 BTU/lbm feedwater.
Total heat input: 1226.2 - 262.25 + 91.7 = 1055.6 BTU/lbm
Total work output: 77.6 + 128.6 = 206.2 BTU/lbm = 19.5%
Total heat rejected: (1104.5 - 88.0) * 0.836 = 849.8 BTU/lbm = 80.5%
(This doesn't add up, I'm missing .4 of a BTU someplace. Ah, well.)
So it would appear that nearly 20% efficiency is achieveable. Even
the rejected heat is good for many purposes, such as space heat or
domestic hot water. It could be a useful system.
I've read, that in the hay-day of steam power, tripple expansion engines were considered to be the most efficent. It makes sense seeing how much work the condenser sheds. Are turbines more efficent? (Or is that what the above discussion assumes.)
Piston expanders are plagued by heat-transfer problems, where hot incoming steam heats the cooler cylinder walls, and then re-heats the cooled steam after expansion. The smaller the temperature drop in any given stage, the smaller the heat transfer losses will be. Large physical dimensions help too. Turbines have advantages, because they are steady-state, unidirectional-flow machines; the hot end is hot, the cool end is cool, and hot and cool fluid never cross the same parts. (Small turbines, I understand, have serious problems with viscous losses. It's 6 of 1, half-dozen of the other.)
Well, I understand all that, but reviewing such proposals in detail is something I get paid (well) to do, and I don't have time to pick through this one in detail. However I think it misses the point, which is that the sun is, for all practical thermodynamic purposes, at infinite temperature, and its energy is nearly 100% convertible to other forms of free energy if you can deal with the high temperatures. The closest that has been done so far is a system built out west (naturally) where a large field of steerable mirrors focus a bunch of sunlight on a "solar furnace" in a tower, which runs at ca. 1000 C or so (and that's still peanuts compared to the Stefan-Boltzman temperature of sunlight). Its been a while since I read about this system - I'm not even sure that water is used - one of the alkali metals would be a better working fluid. The material problems are enormous, of course.
You're thinking of Solar 1, the solar power tower concept. It suffered from thermal cycling problems when clouds crossed the mirror field, I understand. It is being refitted with a molten-salt heat storage system and will re-open as Solar 2, if I am not mistaken. Temperatures were quite a bit lower than 1000 C, however; I believe I saw figures i the mid-hundreds Fahrenheit, but I wouldn't swear to it. I bet there are numbers on the WWW but I have no time to do a search now. Temperature of the solar surface is about 5700 Kelvin.
I did not read or take the time to understand the thermodynamics (Asuume I could remember enough of the basics to do so, which is iffy) but in reading Rane's comments I found a thread of my own thinking. Why is efficiency important? A big problem with Solar power is that it is not particularly reliable (in most places where power is needed). If this item was really not intended to address this type of issue, then that's fine, but is thus reduced to a mere academic exercise.
Efficiency is important because there is limited sunlight in any given area - about 1300 W/m^2. So it would be worthwhile to maximize efficiency. Even more important is return on investment. Solar cells cost more, over their lifetime, than the power produced would sell for.
More to the point, efficiency is important (relatively) because mirror-area is one of the costlier parts of a system. The point of the exercise was to try to get an idea of what is possible, and it appears that the sunlight falling on the roof of a home is more than capable of powering everything inside.
OK, granted. I wasn't poopooing the idea of going for efficiency. It's just that we need to limit such systems to backup duty until we can develop a better storage system. We are doomed to be hooked up to power grids for our power, even if solar is 80% efficient.
That is a point I make very frequently in discussions with people who think we should scrap everything we're using now and switch to "renewables". They're not quite ready for prime time. OTOH, with some clever engineering, they may be made suitable for many uses and economically attractive even at current prices. It's that engineering that I'm taking a look at (and I'm not posting my full analyses here, thanks for asking).
I get a magazine called HOME POWER. It's quite unique in its treatment of the subject of the solar power home and I would recommened to anyone interested in the subject. They also have a CD ROM which contains every- thing form issue 1 to about issue 32. I think they have published about 60 issues now. They also have a BBS one can dial into @ 707-822-8640. They tend to be a tiny bit 60'ish hippy mentality but are quite realistic for the most part. (Readers get pretty vocal when the power Co. bashing gets too out of hand. It's a pretty informative, hands-on, level headed magizine and well worth it's cover price.) No. I don't work for HP ;)
I've read "Home Power". Many of the articles are short on important details. One issue this summer had an article on a solar ice-maker, which amused me enough to calculate the coefficient of performance. My best estimate was about 1%.
It's by no means a scientific journal. It is, however quite good when it comes to practical home instillation of solar cells, storage batteries, wind mills and towers, gasoline to electric car conversions, biogas generation, hydrogen gas generation and usage, DC to AC inverters, battery charging, line synchronous inverters, cogeneration, meeting the NEC for DC powered homes, solar ovens, etc. Most of their material is submitted buy poeple who built solar / wind / hydro powered homes / systems and share their experiances through this magazine. They also run fairly involved tests on various peices of equipment and write up their findings. I find it to be the most relevant, up to date material on the subject of generating power at home using the sun, wind and water.
The COP of a solar ice maker is rather irrelevant. The energy is free and the ice is nice.
Re #29: True, it isn't scientific. However, where it does give good figures, it shows that one's money is usually better invested elsewhere. A feature article about one couple's big investment in wind turbines was accompanied by an editorial slamming utilities on the flimsiest grounds. The letters in the next issue took the editors to task. Moral: If you're on the grid, your alternate-energy investments are best made in commercial-sized units maintained by a company devoted to the enterprise. Household-sized units are not competitive. Re #30: Again, the economics come to bear. If it would be cheaper to make ice some other way (*any* othe way), "free energy" starts looking pretty costly. At $500 invested to make 10 pounds of ice per day, it wasn't very impressive to me. A more interesting exercise would be to find a way to energize an RV refrigerator with sunlight and see if it is cheaper than, say, propane or kerosene in some area of the world. If it can fly on its own economic merits, then it is something worth pursuing. Otherwise it's just a hobby or science-fair project.
America is full of expensive inefficient gadgets that do nothing more than make people happy. There appears to be a market for such gadgets, meaning that they are "efficient enough", or even that that is not the point.
Like I said, a hobby or a science-fair project.
Yes, slamming the utilities was one of HP's weaker moments. I was happy to see the swarm of letters against the slam in the following issue and Richard admitting that he had been out of line. Solar cells, windmills, make sense where it costs $15,000 per mile to bring power into your remote home several miles away from the nearest power lines. It also makes sense in some third world situations where power is very unreliable, if available at all.
No, not the "hobby or a science-fair project". Consumer goods.
I'd say they're consumer goods where the grid doesn't reach; they improve quality of life. Where you've got the grid, they don't give you any more service (usually), they just cost a lot of dough. This isn't the case in some places. On islands where diesels are the current source of electricity, wind generators are perfect as additional generating capacity; the high cost of fuel makes it cost-effective to offset it, and the diesel can track the variations in output of the wind generator. They complement each other well. What I'm still waiting to see is the solar technology that complements other technologies for industry or the home well enough that it makes it cost-effective to make the all-alternative-energy house even where the grid reaches. When that happens, the technology will have *arrived*.
It's just about there when it comes to heating. Solar water and space heating are quite effective, even in Michigan's climate. There will always be the problem of cloudy days where the solar flux is too low and alternative heat sources are needed. Whatever happened to that Co. that was in the news a few years ago? The one being built here in MI that was suppose to make cheap, 80% efficent, solar cells that one could roof their house with for about the same price as shingles? Another pipe dream / scam? I once worked for a guy who had this dream of making millions of tiny dipoles on a substarte. Each dipole would be resonant to "light". Perhaps millions of log-periodic antennas of that size would be better ;)
80% efficiencient solar cells... WHAT???? First I've ever heard of such a thing. They probably went out of business when they ran up against reality.
Re #37: You may be thinking of Energy Conversion Devices, Inc. It's one of Stanford Ovshinsky's babies. FWIW, they were producing stuff that was dirt-cheap, not horribly efficient (max theoretical efficiency of a silicon cell is 20-something percent, I believe). If it is what I believe I remember it was, it was an amorphous cell deposited onto stainless steel sheet, and I actually have some of it (it doesn't work for beans).
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- Backtalk version 1.3.30 - Copyright 1996-2006, Jan Wolter and Steve Weiss